Anyways, of course you can take those coins as when you're running Bitcoin you're strictly speaking not signing a TOS and nobody ever owned those coins.
What people keep private are signing keys for a transaction output. But if you found the key independently, they should be yours too.
My guess is that the court would recognize the first owner as the “real” owner, especially if they can show that they controlled the address up until some point.
In a similar vain condictio indebiti is a principle in maybe jurisdictions, where a receiver of a wrongful payment is required to return it. Even if the payment is made with crypto, the principle would probably stand if it’s practical to enforce.
If a company accidentally sent a bundle of cash via registered mail to the wrong person, the recipient (if known) would probably have to return that cash after a court ruling.
If Coinbase made an erroneous transfer of BTC to one of their customers (whose identities they know), the recipient would probably have to return the bitcoins after a court ruling.
If the recipient of the mailed cash is unknown, or the person associated with the receiving wallet is unknown, then the court could obviously do nothing. But that inability is a function of the knowledge about the parties involved, not of the underlying technologies.
Bitcoin obviously makes it much easier to be unknown to the judicial system. But in my view, for it to be completely not-theft, the recipient should be able to announce publicly what he has done without fear of repercussions.
Similar to how e.g digging up historical artifacts in some jurisdictions is a legal way to gain ownership of something that previously had a different owner.
Re-generating a private key for outputs that have spendable Bitcoins isn't like sending letters to the wrong person.